How the genetic code was cracked

some possible 3-letter codes
The structure of DNA, solved in 1953, set off a race to crack the genetic code. How do sequences of 4 nucleotides code for sequences of 20 amino acids? This coding problem lies at the heart of molecular biology. Physicist George Gamow of Big Bang fame contributed the first guess: Spaces between neighboring nucleotides might fit individual amino acids, directly templating protein assembly on the DNA. In Gamow's solution, each nucleotide must contribute to defining two amino acids–an overlapping code. The numerology looked good (there were exactly 20 possible combinations), but Gamow's solution turned out to be wrong: In 1957, Sydney Brenner devised a simple test that disproved this and all overlapping triplet codes. The true code was soon cracked based on beautiful frameshift experiments by Crick et al., and by analysis of proteins synthesized from artificial RNAs.
Supplements: Gamow's guess, Brenner disproves Gamow and all overlapping triplet codes, the decisive artificial RNA experiments

Elucidation of the RNA interference trigger

The discovery of RNA interference revolutionized the way we determine the role of a gene.  The gene silencing phenomenon has been shown since the early 1990s when introduction of sense or anti-sense RNA could cause a reduction of endogenous messenger RNA.  In 1998, Fire and Mello provided an explanation for the previously reported silencing effect.  Their seminal paper shows that it was not ssRNA that silenced the endogenous RNA, but in fact dsRNA.  So how did ssRNA cause silencing in previous reports?  It is believed that their ssRNA preparations were contaminated with complementary RNA!  Fire and Mello overcame this by extensively purifying their RNA.  Indeed,  they showed that ssRNA was consistently found to be 10 to 100 fold less effective than double stranded.  To this day biologists continue to make great strides in understanding the roles of genes due to this discovery.
Nobel prize article and image credit 

A defective protein causes a disease

It is obvious now that defects in proteins, normally because of mutations in the DNA, cause many diseases, but it was not so evident in 1949.

Linus Pauling and his collaborators knew that only deoxygenated blood contains the sickle shaped erythrocytes (see picture) characteristic of sickle cell anemia, which lead them to the hypothesis that hemoglobin was involved in this problem.

They showed that hemoglobin from patients suffering from sickle cell anemia is different (has different electrophoretic mobility) to the “healthy hemoglobin”. In addition, they found that people with sicklemia, a less severe version of the disease, contain both forms of the protein. This was proof of a change in a protein causing a disease!
More important that the actual experiment, are the conclusions derived of it. Not only this was the beginning of “molecular medicine”, but the genetic discussion in the paper is groundbreaking.
Image credit
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How does the environment affect evolution? Waddington's example of genetic assimilation

Conrad Waddington, throughout his long and varied career as a developmental biologist, was foundational to several aspects of modern evolutionary theory, such as epigenetics, developmental canalization, and genetic assimilation. One of the great puzzles of evolution is how organisms can become so specifically and heritably adapted to their environment. Random genetic mutations can sometimes serve as the sole explanation, but not always. Through several rather cleverly simple experiments, Waddington demonstrated that phenotypes elicited by a specific environmental cue (such as heat shock or ether treatment) could be "assimilated" into the genotype. This means that the phenotype could eventually be expressed even if the corresponding cue was absent. He argued that this phenomena, which he witnessed in Drosophila in less than 30 generations, could play a powerful and vital role in evolution.
Background paper: Genetic assimilation of cross-veinless phenotype (1953)

Barbara McClintock discovers "jumping genes"

Transposable elements (TE) are DNA sequences that “jump” from one location in the genome to another.  McClintock’s work not only showed that sequences can move, but also that this movement across the genome can create and reverse mutations as well as alter genome size, all during various stages of cell development.  McClintock conducted standard genetic self-breeding experiments causing broken chromosomes and noted unusual color patterns in the offspring, to understand the cause of this variety she compared the chromosomes of each generation with that of the parent and found that certain sections of the chromosomes had switched their position.  At first her discovery was met with skepticism because it went directly against the popular theory at the time that genes were fixed in their chromosomal position but McClintock’s work was rediscovered through work in bacteria a decade later and earned her a Nobel Prize in 1983.
Supplementary paper:  Junk DNA as an evolutionary force

How DNA synthesizes itself

After the structure of DNA was elucidated by Watson and Crick, one of the next burning questions was how is it replicated? The aforementioned individuals contributed the first hypothesis: that DNA replication is semi-conservative. That is, each strand of DNA serves as a template for a newly synthesized strand. A second was the conservative hypothesis, that the entire DNA molecule serves as a template for a new DNA molecule. And finally, the dispersive hypothesis proposed by Max Delbrück argues that a mechanism exists that would break the strand every so often and attaches a new strand to the old one. To test this, Matthew Meselson and Franklin Stahl, in an incredibly elegant experiment published in 1958, grew E. coli first with 15N then with 14N and allowed to divide. They periodically extracted the DNA and centrifuged the DNA in a cesium chloride density gradient. The results were obvious. Through cell division, half of the DNA was replaced with new DNA, favoring the Watson and Crick hypothesis that DNA replication is semi-conservative.
Image: from the Meselson-Stahl Experiment Wikipedia page.

Lenski "replays" evolution in longterm E. coli populations

Richard Lenski's longterm evolution experiments on E. coli are a hallmark example of evolutionary biology. Lenski and colleagues have maintained 12 parallel lines of E. coli for 50,000 generations now. Initially, these E. coli populations were founded by clones, and over decades, researchers have watched evolutionary dynamics on a scale observable in real time.

This particular paper, published in 2008, describes the acquisition of a novel phenotype - the ability to metabolize citrate in addition to glucose as an energy source.

Lenski's experiments on the evolution of citrate use are particularly elegant for the following reason: The lab maintains frozen samples of the E. coli populations at time points throughout the history of the populations. These samples are not growing (and therefore not mutating) while frozen, but can be pulled from the freezer and reconstituted. This allowed researchers to go back to previous timepoints in the evolution of this phenotype and "replay evolution" to see if the same phenotypes arise again ...

Rats with an intense social life do not become drug addicts.

Between 1950 and 1975 many studies had shown that rats that have had morphine previously administrated become addicted to the drug, since later on they choose to drink a morphine solution when water is also available.

Bruce Alexander had a problem with those studies as those rats had been kept in “small, solitary metal cages” which, he thought, could influence the results. He therefore designed a Rat Park: an open-topped cage with sawdust on the floor and multiple toys (including a climbing pole) and friends to play with (see image).

He performed experiments to test morphine addiction in rats that had been either isolated or living in Rat Park and saw that social rats did not become addicted to morphine. He concluded that it was the “spatial confinement, social isolation, and stimulus deprivation” what made them drug addicts rather than the drug itself.

The Discovery of Passive Immunity

After vaccinations were shown to be successful at protecting individuals against pathogens, researchers next wanted to know how it worked. To address this, immunologists Emil Behring and Shibasaburo Kitasato teamed up to work together in Germany. They injected serum from immunized rabbits into the abdominal cavity of six mice. After twenty-four hours they infected the treated and untreated mice with virulent tetanus bacteria. All of the control mice died, but the treated mice survived and showed no sign of infection. This important discovery showed that the substances that impart the protection appear in serum following immunization and that immunity can be passively acquired. Behring eventually received the first Nobel Prize in Physiology or Medicine for this work. 

Mummy Says Beer is Healthy

A chemist, a doctor, and two anthropologists walk into a bar. Mark Nelson, Andrew Dinardo, Jeffery Hochberg, and George Armelagos discovered that had they been Ancient Nubians, their beer would have contained antibiotics. This would have been nearly two thousand years before the "first" discovery of the antibiotic penicillin by Alexander Fleming which won him his Nobel Prize. Using acid extraction and mass spectroscopic characterization, they investigated reports on bones that when test with UV light produced yellow-green fluorophore deposition bands indicative of tetracycline. They rejected the claims that the exposure was postmortem and proposed the contents had been ingested over a long period of time. According to Nelson, this ancient population did not accidentally mass produce the antibiotic. It is believed that the nutrition and pharmacological effects of the fermentation was purposefully done.

Fleming's discovery of penicillin, aka "mould juice"

Alexander Fleming is famous for, among other things, his discovery of penicillin. As the old tale goes, some of his bacterial cell culture plates became contaminated from the air, and he came to work the next morning to realize that a contaminating mould was secreting something which seemed to be killing the bacterial cells. He carried out a series of careful experiments characterizing the mould (determined to be of the genus Penicillium) and the effects of its secretion as an anti-bacterial agent. His initial article received little attention, and mass culturing the mold and extracting the penicillin itself was difficult. He gave up trying and the project was quickly picked up by Howard Florey and  Ernst Boris Chain. With funding from the US and UK in WWII, Florey and Chain were able to figure out how to produce mass quantities. Fleming, Florey, and Chain shared the Nobel Prize in 1945 for this discovery.

Natural selection changes allele frequencies

Researchers had known that marine sticklebacks which colonized freshwater, an event that had happened naturally multiple times, will gradually lose their bony armor plating. This was presumed to be due to an increase in fitness sans armor when living in a freshwater environment, but armor is been maintained in the marine environment due to a fitness advantage. The armor is controlled mainly by a single locus, Eda. The allele of Eda which causes decreased armor is ancient and is segregating at low frequency in the natural marine populations.

Barrett, Rogers, and Schluter trapped marine armored sticklebacks and transplanted them into freshwater ponds. They observed the expected loss of armor in the now-freshwater fish. They tracked allele frequencies at the Eda locus as well as phenotypes of the fish over time to determine if the loss of armor was the result of positive selection. The combination of natural founding populations, natural environments, and phenotype & allelic correlations makes this experiment a particularly simple and elegant example of modern evolutionary research.

Jellyfish enlighten our view of cells.

Fluorescent North American jellyfish species contain green fluorescent protein that absorbs blue light from the environment and in turn produce a green luminescence. Researchers became interested in this protein in particular, compared to other fluorescent methods, due to its ability to fold and function without the need for additional enzymes. The protein’s gene can be delivered into novel genomes using viruses or a number of other techniques to cause it to be expressed in certain cell types. This gives us the ability to follow cell lines as they develop and study gene expression, both of which were too small and difficult to identify before this development. This work resulted in a Nobel Prize in Chemistry in 2008 and has continued to be developed into a multitude of colors and inspired work and development of other proteins that emit near-infrared light that can be more easily detected through tissue.

Hawk moths use their genitals to jam bats’ sonar.

When we think of sonar jamming we picture a modern day military tactic, yet hawk moths have long been using the same strategy in order to also counteract the radar of their enemies as well. Bats rely on ultrasonic echolocation to see their surroundings and locate prey, hawk moths are able to recognize the bat’s sonar and respond by rubbing their genitals against their abdomens to create a responding ultrasound that is meant to startle or hinder the bat’s echolocation. Moths have a history of unique adaptation to counter predation by bats but the work by Barber showed that there are even differences within the same species between the sexes in what mechanism is used to get the same ultrasound effect. The ultrasound response by hawk moths is only used near the end of the bat attack sequence and could hint at it being a last line of defense among an arsenal of already existent antipredator adaptations.

The Miller Experiment: Lightning may have created the building blocks of life.

In 1952, Stanley Miller conducted an experiment to see if he could create organic compounds from inorganic compounds.  To do this he tried to create conditions similar to what earth might have been like before life emerged on a primitive Earth.  A mixture of water vapor, methane, ammonia, and hydrogen in the presence of electrical discharge (lightning) caused the water to turn red.  This color change was due to the presence of organic compounds.  The compounds generated by this method were ran on paper chromatogram and some were determined to be amino acids when compared to chromatograms of known compounds.  Following Miller's death, examination of sealed vials revealed that Miller actually generated many more amino acids than originally reported. 

Don’t go tanning at 4 in the morning (at least if you are mouse)

Even though the Nobel Chemistry Price committee rewarded Dr. Aziz Sancar for his DNA repair work, he is focusing in studying the circadian clock now. One of reason for this change is that genes involved in nucleotide excision repair genes (NER, a key pathway in DNA repair that he studied) have different expression levels throughout the day and are controlled by the cell’s circadian rhythms. In this paper, he shows that behavior for XPA, an important NER protein. In addition, he correlates its expression with the odds of mice getting skin cancer depending on the time they are exposed to UV light. Unfortunately, these results are hard to extrapolate to humans as we don’t have the same circadian clock as mice, so I can’t really tell you what time of the day is best to go sunbathe!